This invention relates to the removal of oxides of nitrogen (NO.sub.x) from hot or cold gas mixtures. More particularly, this invention relates to the removal of oxides of nitrogen from combustion gases, particularly exhaust gases from internal combustion engines.
A common form of pollution exists where automotive traffic is heavy and meteorological temperature inversions are common. This form of pollution is customarily referred to as "smog" although it is not a true mixture of smoke and fog.
Visually, smog does not resemble fog but is more like a heavy haze. It has a distinct ordor and is irritating to mucous membranes, particularly of the eyes. It markedly reduces visibility, causes ozone formation and brings about serious damage to certain species of plants. The principal source of smog is the exhaust gas from internal combustion engines. However, flue gases from industrial sources also contribute to the formation of smog conditions. Smog is the product of sunlight-induced atmospheric reactions of organic compounds with oxides of nitrogen such as the rection of hydrocarbons and NO.sub.x as produced in an internal combustion engine.
The hydrocarbons in exhaust gases arise from the incomplete combustion of the fuel. The nitrogen oxides result from the fixation of nitrogen during combustion and are formed during all phases of operation in the internal combustion engine. Their concentration in the exhaust gases varies from low levels, i.e. 20 p.p.m., at idle motor conditions, to high levels, i.e. 1000 p.p.m., or more, during acceleration.
Most of the total oxides of nitrogen emitted in automotive exhaust are present as nitric oxide (NO) which is readily converted to nitrogen dioxide (NO.sub.2) in the atmosphere. The dioxide absorbs ultraviolet radiation in sunlight and is dissociated into a nitric oxide molecule and an oxygen atom. The oxygen atom enters into chemical reactions with the organic constituents of exhaust gases, particularly the hydrocarbons. Furthermore, the photolysis of nitrogen dioxide (NO.sub.2) appears to be important in ozone formation. Accordingly, it is desirable to remove NO.sub.x from flue gases and automobile exhaust to control air pollution.